1. Field of the Invention
The present invention relates to an image forming apparatus such as electrophotographic copying machines, printers and facsimile machines, and a fixing apparatus provided in the image forming apparatus.
2. Description of the Related Art
The thermal roller type fixing apparatus has been widely used for image forming apparatuses such as electrophotographic copying machines, printers and facsimile machines. The thermal roller type fixing apparatus comprises a heat roller and a pressure roller that is held in contact with the heat roller under pressure, wherein a recording medium (hereinafter called "sheet") such as recording papers and OHP sheets, carrying unfixed toner image passes through a contact area, or a nip area between the heat roller and the pressure roller, thus causing the toner image to be fixed on the sheet.
Under the pressure of increasing demands for quick printing and low power consumption in recent years, a fixing belt type fixing apparatus was proposed as disclosed in JP-A-06-318001.
This kind of fixing apparatus has a heat source placed at a distance from the nip area and uses a low heat capacity fixing belt in order to shorten the time needed for raising the temperature. It results in a severe temperature drop in the part of the fixing belt which comes in contact with the sheet at the nip area.
Consequently, a fixing apparatus disclosed in the JP-A-09-138599 has a pair of rollers that supports and rotates the fixing belt, wherein one of rollers is equipped with a heat source, while the other of rollers contacts the pressure roller under pressure across the fixing belt to constitute the nip area. The fixing apparatus further has two temperature sensors, one on the roller equipped with the heat source and the other on the pressure roller with no heat source that directly contacts the nip area, wherein the two temperature sensors provide temperature data for temperature control.
A problem, however, is a complexity in temperature control based on the temperatures of the roller equipped with the heat source and the pressure roller that is contacting the fixing belt and constituting the nip area, which are measured by the sensors, respectively.